In many technical processes it is advantageous to be able to measure the different parameters of the process in real time to examine, monitor and control said processes without disturbing the course of the process or state of the process itself. Optical measuring methods which are based on determining the state or properties of the target on the basis of the electromagnetic radiation (later shortly radiation) obtained from the target, offer according to their basic nature a possibility for non-intrusive measurements. Methods based on conventional physical probes, such as measurements using thermocouples (temperature measurement) or different methods based on sampling (e.g. concentration measurements) always disturb the target to be measured to a certain degree. When compared to traditional physical probes, by means of optical methods it is in many cases possible to conduct the measurements with a significantly better temporal and spatial resolution. It is also advantageous to use optical methods in connection with such processes in which the use of physical probes is impossible or difficult because of high temperatures prevailing in the process or other conditions hostile to the physical probes.
Optical measuring methods can be divided into different classes by means of various criteria. If the instantaneous spatial resolution attained by means of optical measuring methods is used as a comparison criterion, it is possible to divide the methods to non-imaging and imaging methods on the basis of this criterion. The basic differences between these methods are shortly described in the following.
In imaging methods the detector that is utilized is a suitable two-dimensional, spatially resolving detector (hereinbelow shortly matrix detector), wherein the electromagnetic radiation obtained from the target by means of suitable front optics is collected and focused on the light-sensitive screen of the aforementioned detector. In the wavelength range of visible light the matrix detector may be for example a so-called CCD or CMOS camera. The screen of the aforementioned detector is composed of separate small light-sensitive detector units (hereinbelow pixels), of which each pixel collects the radiation transmitted by a fixed part of the target according to the imaging properties of the front optics. When the radiation signal collected/detected by the aforementioned pixels during a predetermined integration time is changed into an electric form in such a manner that the pieces of information contained in different pixels are kept apart from each other, spatially resolved information is obtained from the area of the target that is imaged in the aforementioned manner, said information being collected from the entire area either exactly or substantially at the same time, depending on the structure and operating mode of the used detector.
In non-imaging methods the radiation is typically detected by means of only one such detector, such as a photo diode or a photomultiplier tube in which the radiation entering on the light-sensitive or radiation sensitive surface of the same produces an electric signal that cannot be traced as a function of the spatial location of the screen of the detector and thus, more specifically, as a function of the location of the radiation signal collected by said detector from the target. Thus, the properties of front optics are typically selected either in such a manner that radiation signal is collected simultaneously on the entire area of interest of the target, or alternatively in such a manner that a smaller measuring point detected by the detector in the target is temporally scanned to different sections of the target to obtain spatially resolved measurement information. In the latter case the information obtained from different sections of the target is, however, measured substantially at different moments in time, which is a significant restriction in cases where rapidly changing processes which are spatially heterogeneous are used.
The rapid development of detector technology and especially the development of matrix detectors both in the visible light range (wavelength range of approximately 300 to 800 nm) and in the ultraviolet range (<300 nm) and in infrared (>800 nm) has enabled a strong increase in the use of imaging measuring methods in research, monitoring and control operations of different processes applied in the industry. Together with the development of computer and image processing technologies enabling a more efficient processing of image information, the aforementioned matrix detectors nowadays make it possible to develop imaging measuring methods which function substantially in real-time.
Imaging optical measuring methods can be further divided into non-spectroscopic and spectroscopic methods. In non-spectroscopic imaging methods which typically include most of the conventional machine vision methods (the parameter to be measured for example the size, location or position of the target), the electromagnetic radiation obtained from the target is not divided especially according to the wavelength of radiation, but the radiation is detected typically only in one wavelength band. This wavelength band may be determined for example according to the radiation used for illuminating the target, and/or according to the natural spectral operation range of the matrix detector used in the measurement. It must be noted that in this text the term optical does not refer solely to the wavelengths of visible light (approximately 300 to 800 nm), but radiation with a substantially shorter (ultraviolet range) or longer wavelength (infrared range) than visible wavelength is also possible.
In spectroscopic methods, i.e. in methods based on spectral resolution the radiation obtained from the target is, however, divided into two or more spectral bands differing from each other, wherein by comparing and/or combining the signals/images measured at different wavelength bands it is possible to determine parameters of interest in the target, such as local temperature, or local concentration of a particular component of interest. More commonly used spectroscopic methods whose basic principles are known are for example two-color or multicolor pyrometry, by means of which it is possible to determine the temperature of the target on the basis of the electromagnetic radiation emitted spontaneously by the target. By means of a suitable external stimulus (e.g. laser light or so-called spectral lamps) it is also possible to conduct measurements based on optical absorption, or elastic (e.g. so-called Mie scattering from particles/droplets) or nonelastic (e.g. so-called fluorescence or Raman scattering) scattering of radiation, such as concentration measurements. The principles of the above-mentioned spectroscopic methods, which in this context include pyrometry as well, are generally and widely known, and therefore they will not be discussed in more detail herein since they do not form a part of the actual invention.
To implement the aforementioned spectroscopic methods, it is often necessary to use spectrally resolved information measured at least on two wavelength bands to define the parameter of interest in the target. In imaging methods this usually means that the spectral bands resolved from each other by means of a beamsplitter/beamsplitters and different optical filters are guided to a separate matrix detector each, or alternatively all spectral bands are guided to the same matrix detector in such a manner that the signals produced by them can be distinguished from each other.
Naturally, in spectrally resolving imaging measurements intended for industrial conditions and applications, the above-described use of several separate matrix detectors is problematic in that respect that said measuring devices have a complex structure and they are expensive. Therefore, a more interesting solution in view of industrial applications is the use of a single matrix detector for detecting all spectral bands to be measured and at the same time the attempt to reduce the number of optical components required in resolving said spectral bands and focusing them to the light-sensitive screen of the detector as well as to minimize the adjustments required in positioning these components, i.e. to simplify the structure, implementation and use of the measuring device. In industrial conditions another significant factor is also the compact mechanical structure of the measuring device which is attained in the above-described manner and which endures external conditions well.
The following is a description of known solutions that can be used with spectroscopic measuring methods and which enable imaging spectral resolution.
Patent publication U.S. Pat. No. 4,413,324 discloses three different ways of implementing spectrally resolved imaging measurement by means of matrix detectors. More precisely, the measurement in question is an imaging pyrometric two-color temperature measurement of a target, conducted by means of two measurement wavelength bands differing from each other. The first method described in the aforementioned publication is based on the use of optical filters of two different types placed in front of the screen of one matrix detector (camera), the spectral bands of the filters differing from each other. The aforementioned filters, the size of each of them advantageously corresponding exactly to the size of a single pixel in the detector, together form a continuous mosaic filter that covers the light-sensitive screen of the matrix detector entirely. A second method disclosed in the same publication is based on the temporal measurement of spectral bands at different moments in time by using a disc to be rotated in front of one matrix detector, said disc being composed of two different optical filters to attain spectral resolution. A third method disclosed in said patent publication is based on the act of dividing the radiation attained from the target into two spectral bands differing from each other, each band being guided to separate matrix detectors of their own. It is characteristic to all the methods disclosed in the patent publication U.S. Pat. No. 4,413,324 that two separate spectral bands are used in them and that the detector/detectors uses/use the entire imaging area for only one measuring method.
Patent publication U.S. Pat. No. 5,963,311 discloses another type of a device suitable for imaging two-color pyrometry, in which the radiation received from the target is divided into two parts, which parts are guided through different optical filters further to a matrix detector in such a manner that the images corresponding to different filters and representing different wavelength bands, which both correspond to the same area imaged from the target, are formed on the screen of the matrix detector adjacently with respect to each other. In the method disclosed in said publication the radiation received from the target is first used to form an image in the so-called intermediate focus of the optics, from which it is imaged on the screen of the actual detector. The use of the intermediate focus enables the adjustment of the magnification of the two adjacent images formed on the screen so that the magnification is equal in both images, as well as a better control of scattered light between said images.
Patent publication U.S. Pat. No. 5,225,883 discloses an arrangement suitable for imaging two-color pyrometry of a stationary or moving/flowing target. Similarly to the method disclosed above in the patent publication U.S. Pat. No. 5,963,311, in this case the radiation received from the target is divided into two parts, which parts are guided through different optical filters further to a matrix detector in such a manner that the images corresponding to different filters and representing different wavelength bands, which both correspond to the same area imaged from the target, are produced on the screen of the matrix detector adjacently with respect to each other. When compared to the solution disclosed in the publication U.S. Pat. No. 5,963,311, the solution presented in the publication U.S. Pat. No. 5,225,883 does not apply an intermediate focus in the adjustment of magnification, but in the other optical arm an optic component with a suitable refractive index is utilized to compensate the path length difference between the optical arms corresponding to the images, thus enabling the focusing of said two images on the screen of the detector by means of magnification which is exactly equal in both images.
It is characteristic to all above-presented imaging solutions that enable a substantially simultaneous spectrally resolved measurement on several spectral bands that the division and/or filtering of the radiation attained from the target into spectral bands that differ from each other takes place in such a manner that said process is conducted in the same way for the area imaged from the entire target, and the entire imaging area of the matrix detector/matrix detectors is thus used for the same spectroscopic measurement, such as two-color pyrometry. Thus, it is a drawback and a considerable restriction of the aforementioned methods in imaging measurements based on spectral resolution that they can be advantageously used only for spectroscopic measurements of one type at a time without changing or adjusting the optical components. Furthermore, the filters selected for a particular spectroscopic measurement are not optimally suitable for mere visualization of the target, or other non-spectroscopic measurements.
Moreover, it is a problem in the above-presented known solutions that it is necessary to use several optical components therein to divide and/or filter the light obtained from the target into different spectral bands and to focus it to the matrix detector, which said components must, in most cases, be adjusted and focused with great accuracy with respect to each other and/or the matrix detector. Especially in those known solutions in which the images measured on different spectral bands that correspond to the same location in the target, are projected separately (U.S. Pat. Nos. 5,963,3115, 5,225,883) next to each other on the screen, the picture elements corresponding to a particular part of the target on different wavelength bands in the matrix detector are located far away from each other. This complicates a reliable mutual identification of said picture elements, and it sets special requirements in view of focusing and adjusting said optical components, so that magnifications of images measured on different wavelengths become equally large in size. Correspondingly, a preferred embodiment of the mosaic filter disclosed in the patent publication U.S. Pat. No. 4,413,324 requires that each single filter is positioned accurately to correspond to one or several pixels of the detector. This is technically challenging and therefore expensive especially in cases of smaller production batches.
In view of the present invention, the solution disclosed in the patent publication U.S. Pat. No. 5,225,883 can be considered as the closest state of the art solution among the above-presented prior art solutions, and in said publication the suitability of the solution for measurement of moving or flowing target is also emphasized. Said publication does not, however, in any way mention the possibility of utilizing the movement of the target for recording spectrally resolved information and further for conducting spectroscopic measurement in a manner intended by the invention disclosed in the present application.